Product containing high density skins on a low density core and method of manufacturing same

ABSTRACT

A fiberboard is manufactured having relatively high density skins on a relatively low density core by including a skin forming chemical in at least the surface fibers of a consolidated mat, having a density of less than 35 pounds per cubic foot, and then hot-pressing the consolidated mat at a temperature of at least 525° F. to form a board having high density surface skins.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of our co-pending application Ser. No. 739,184 filed Nov. 5, 1976, herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a fiberboard panel having a relatively low density core or central portion and having relatively high density skins or surface portions. More particularly, the present invention relates to a man-made board having high density skins or surface portions as an integral part of a relatively low density core wherein the high density skins are produced by contacting the surface fibers of a relatively low density consolidated board with a skin forming chemical, and thereafter hot pressing the board in a post-press or second pressing operation at a temperature of at least 525° F. to densify the surface portions into smooth, stiff, hard, dense skins.

BACKGROUND OF THE INVENTION

The process of the present invention relates to a "post-press" or second press operation for creating thick, hard, integral surface portions on at least one and preferably both surfaces of a man-made fiberboard substrate. A handleable mat is first manufactured by consolidating a cellulosic fiber-containing composition to form a consolidated fiberboard having a density of less than 35 pounds per cubic foot. The handleable mat can be consolidated by drying a mat formed by depositing a water slurry of fibers onto a water pervious support member, as is common in the manufacture of insulation board, or the mat can be consolidated under heat and pressure until substantially dry in a first press operation. This consolidated mat is thereafter "post-pressed" to form dense surface layers for stiffness and strength, while retaining the relatively low density core or central portion. The density of the core is substantially predetermined when the mat is initially consolidated. For the purpose of the present invention, a "post-press" or "post-consolidation" pressing operation refers to a press step performed on the handleable mat after consolidation thereof.

It is necessary to form the fiberboard of the present invention in two separate operations. The first operations forms a consolidated mat and sets the core density while the second or "post-press" operation creates the denser surface portions. It is quite surprising that a "post-press" or second pressing operation is effective in densifying a surface of a consolidated mat while leaving the density of the core material substantially unchanged. Prior to the present invention, it was thought that the qualities and physical characteristics of a board would be finally determined by the conditions under which the board is initially consolidated and by the raw materials used to form the board, because of the bonding occuring during consolidation.

Much of the bonding necessary for cohesion and strength in a man-made board occurs during the consolidation of the board during hot-pressing. Prior to drying the board made by the wet process, the board is weak, but the bonding forces created during the final consolidation are generally quite powerful. Generally the same bonding effect occurs in the final consolidation of a board made by the dry process. Prior to hot-pressing, the loosely disassociated cellulosic fibrous product is quite weak, but after hot-pressing the mat into its final configuration, it is very powerfully held together by the bonding which occurs during hot-pressing. Hot-pressing during consolidation causes a welding or coalescing of the cellulose fibers at the surface of the product so that the surface portion consists of wood remade in modified form. It is very difficult to densify or restructure the surface of a consolidated product without again destroying these fiber-to-fiber surface welds. It is especially difficult to densify only a surface portion of a consolidated board in an amount sufficient to substantially increase the strength and stiffness of the product while retaining the lightweight core material to provide a lightweight product. The process described herein accomplishes these results.

A prior application, assigned to Masonite, Ser. No. 537,254 filed Aug. 23, 1974, discloses and claims a method of post-press embossing consolidated, man-made boards to effectively and accurately reshape the surface of a completed panel without surface defects such as surface fiber tearing. In accordance with Ser. No. 537,254 filed Aug. 23, 1974, a previously consolidated panel is post-press embossed at a pressure of at least 1000 psi to create a surface design, or to permanently redesign the surface of a consolidated product without destroying the interfiber bonding established at the surface of the board during consolidation. The product thus formed does not include a relatively low density core or central portion.

In accordance with the present invention, it has been found that the surface density and physical characteristics of a relatively low density consolidated mat can be altered in a post-press operation to harden, smooth, and densify the surfaces thereof when at least the surface fibers of the mat are contacted with a skin forming chemical. During the post-press operation, at a temperature of at least 525° F., the skin forming chemical reacts to stiffen and strengthen the cellulosic substrate, such as low density fiberboard, to provide the necessary strength for the intended purpose of the substrate while retaining a relatively low density, lightweight central core of the material. In this manner, the resulting post-pressed substrate is very lightweight while having strength and stiffness properties equal to or better than much heavier boards.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a lightweight fiberboard panel having hard, dense surface skins thereon.

Another object of the present invention is to provide a lightweight cellulosic fiber-containing article having a hard, dense skin on at least one surface thereof.

Another object of the present invention is to provide a lightweight fiberboard, having a central core of material with a density in the range of 10-35 pounds/ft.³, having strength and stiffness properties equivalent or better than hardboard having a density in the range of 45-65 pounds/ft.³

Another object of the present invention is to provide a lightweight man-made board having design-fidelity and paint hold-out properties equal to or better than hardboards manufactured in accordance with existing technology.

Another object of the present invention is to provide a method of providing an integral, structural skin on one or more surfaces of a cellulosic fiber-containing consolidated product by including a skin forming chemical within at least the surface fibers of the consolidated product, and, thereafter, post-pressing the chemically treated consolidated board in a second-pressing operation at a temperature of at least 525° F.

In accordance with an important feature of the present invention, it has been found that particular chemicals will provide hard, dense surface skins to a handleable mat when the surface fibers are contacted therewith and the mat is thereafter "post-pressed" at a temperaure of at least 525° F.

In accordance with another important feature of the present invention, the technology disclosed herein has been developed to provide a low density, strong cellulosic fiber-containing product, for example, fiberboard which has sufficient strength for end uses such as wall panelling, siding, and moldings. The low density, high strength product is produced by first manufacturing a low density substrate having strength sufficient to be handled in manufacture, including a skin forming chemical in at least the surface fibers of the substrate and thereafter post-pressing the substrate to develop dense outer layers or skins on the exterior surfaces.

Surprisingly, it has been found that the skin created by post-pressing a low density consolidated product having a skin forming chemical in at least the outer surface fibers thereof creates a hard, dense surface which, if desired, effectively and permanently reproduces embossing plate details on the surface of the product and creates a smooth outer surface having excellent holdout of coating materials, such as paint. The post-pressing operation restructures the surface fibers contacted with the skin forming chemical to provide exceptional strength and stiffness properties to a lightweight panel.

DETAILED DESCRIPTION OF THE INVENTION Low Density Handleable Mat

In accordance with the present invention, a high strength fiberboard product having hard, dense skins or surface portions thereon is manufactured in two steps. The first step comprises manufacturing a relatively low density consolidated, handleable mat, using either the wet or dry process as known in the manufacture of man-made boards; the second step comprises post-pressing the consolidated mat after first treating at least the surface fibers of the mat with a chemical to form a skin on the surface of the product.

The method of producng a consolidated mat is well known as presently used in producing man-made boards such as hardboard, chipboard, particle board, panelboard, acoustical board, insulation board, and the like. In the wet process, the raw material is uniformly blended in a head box with copious quantities of water to form a slurry. The slurry is deposited onto a water-pervious support member, generally a Fourdrinier wire, where much of the water is removed leaving a wet mat of cellulosic material. The wet mat is then dried to consolidate, as in the manufacture of insulation board, or can be transferred from the pervious support member and consolidated under heat and pressure to form the board. Typically, pressures of from 400 to 500 psi and temperatures up to about 400° F. are encountered in hot-press consolidation of a man-made board manufactured by the wet process. The dry process is similar to the wet process except that the cellulosic fibers are first coated with a thermosetting resin binder, such as a phenol-formaldehyde resin, and are then randomly distributed into a mat by air blowing the resin-coated fibers onto a support member. In the dry process, the mat is pressed at temperatures up to about 450° F. and pressures less than about 1000 p.s.i. to cure the thermosetting resin and to compress the mat into an integral consolidated structure.

The handleable mat produced in accordance with the first step of the present invention is manufactured in a conventional manner, using conventional cellulosic fiber stock. To achieve the full advantage of the present invention, the handleable mat should have a density, after consolidation, in the range of 10-35 pounds per cubic foot, preferably in the range of 15-30 pounds per cubic foot. Panels of varying thicknesses having densities within this range can be produced in accordance with known technology to provide lightweight core materials on which a surface skin can be developed in accordance with the following disclosure.

Handleable mats have been produced having a density as low as ten pounds per cubic foot and a thickness of one and one-half inches. Mats having densities as low as ten pounds per cubic foot are useful as the handleable mat in producing products in accordance with the principles of the present invention. In fact, in accordance with the presention invention, any low density mat (less than 35 pounds per cubic foot) is considered to be handleable if it can survive the trimming, cutting, stacking, packing, shipping, and unloading operations necessary to produce fiberboard. All such handleable mats are useful in accordance with the principles of the present invention.

Skin Development

The consolidated mat containing a skin forming chemical is pressed in a post-pressing operation at a temperature of at least 525° F. to develop a surface layer therein called a "skin", defined as an outer layer having a higher density than a thickness of material over which it is formed. It was found in early investigations that heat and pressure alone (without a skin forming chemical) would not form a thick skin on the surface of a low density panel. Skin thickness was very thin regardless of the temperature and compression. Table I shows that post-pressing alone, without pre-treatment with a skin forming chemical, does not produce a skin.

                  Table I                                                          ______________________________________                                         Effect of Heat Pressure and Post-Pressing                                      On Skin Development                                                                      Final   Den-    Skin    Stiff-                                                                               Breaking                                         Caliper sity    Thickness                                                                              ness  Load*                                  Substrate (In.)   lb/ft.sup.3                                                                            (In.)   lb/in.                                                                               (lb.)                                  ______________________________________                                         fiberboard                                                                     not post-pressed                                                                         0.563   17      0       103   24                                     post-pressed                                                                   fiberboard                                                                     without a skin                                                                 forming                                                                        chemical  0.406   22      0        68   20                                     ______________________________________                                          *The stiffness and breaking load data as set forth throughout this             disclosure, were obtained by breaking 3" × 12" samples over a 10"        span. A load was applied to the center of the span and the center of the       sample was deflected at a constant rate. As the sample was deflected, the      force (load) exerted on the sample was recorded. As the sample is bent,        the force is continuously increased until the sample breaks. During the        initial phases of loading, the force applied is a linear function of the       deflection. The term "stiffness", as used throughout this disclosure, is       the slope (lb/in.) of this linear portion of the load deflection curve.        The term "breaking load", as used throughout this disclosure, is the           maximum load point obtained on the load/deflection curve before the sampl      breaks.                                                                  

Various chemicals were evaluated on the surface of low density mats to determine which chemicals provide a high density skin on the surface of a low density mat when the chemically treated, consolidated mat is "post-consolidation" hot-pressed at a temperature of at least 525° F. The following chemicals set forth in Table II provide a high density skin of at least 0.020 inch thickness on the surface of a low density (10-35 pounds/ft³) mat when the consolidated mat, containing the chemical, is hot-pressed at a temperature in the range of 525°-650° F.:

                                      Table II                                     __________________________________________________________________________     LOW DENSITY, STRESSED-SKIN                                                     SKIN PRODUCERS                                                                 __________________________________________________________________________                               APPLICATION RATE*                                                              (lbs per thousand                                    CHEMICAL       SKIN THICKNESS                                                                            square feet)                                         __________________________________________________________________________     Ammonium Thiocyanate                                                            (Strong Sulfur Odor)                                                                         .030       170                                                  Thiourea (Strong Sulfur Odor)                                                                 .030        85                                                  Acetamide      .020       140                                                  __________________________________________________________________________      *The application rate has no bearing upon the minimum amount of chemical       necessary to form an adequate skin.                                      

                              APPLICATION RATE                                                               (lbs per thousand                                    CHEMICAL       SKIN THICKNESS                                                                            square feet)                                         __________________________________________________________________________     Dicyandiamide  .030       140                                                  Ethanolamine   .035       195                                                  Hexamethylenetetramine                                                                        .030       100                                                  Hydrazine + Formaldehyde                                                       in a 1/1 weight ratio                                                                         .020        90                                                  Ammonium Xylene Sulfonate                                                                     .020       150                                                  Mono-Ammonium Phosphate                                                                       .030       100                                                  Di-Ammonium Phosphate                                                                         .030       100                                                  Ammonium Sulfate                                                                              .025       100                                                  MASONEX* (Trademark of                                                                        .030       150                                                  Masonite Corporation)                                                          Sucrose        .025       100                                                  Boric Acid/Borax                                                               in a 1/1 weight ratio                                                                         .030       150                                                  Linseed Oil + Iron Tallate                                                     (2% by weight of oil)                                                                         .035        75                                                  __________________________________________________________________________      *MASONEX (Trademark of Masonite Corporation) is a hemicellulose extract        derived from wood by steam explosion treatment (digestion) of wood chips       without chemical addition and contains the following components:         

    Protein     .5%    Simple Sugars                                                                   (after hydrolysis)                                                                          35.0%                                         Fat         .5%    Distribution of Sugars**                                                        (after hydrolysis)                                         Fiber       1.0%   Glucose       14.0%                                         Ash         6.0%   Mannose       27.0%                                         Solids      60.0%                                                              Calcium     .5%    Galactose     8.0%                                          Phosphoros  .07%   Arabinose     5.0%                                          Metabolizable      Xylose        46.0%                                         Energy (ruminants)                                                                         890    pH            5.0%                                          Simple Sugars**                                                                            10.0%  Carbohydrates 55.0%                                         __________________________________________________________________________      **The sugars, both before and after carbohydrate hydrolysis, cause skin        formation                                                                

Mixtures of any two or more of the chemicals listed in Table II can also be used as the skin forming chemical in accordance with the principles of the present invention.

The following group of chemicals taken from Table II provide stronger skins when a consolidated low density mat is post-pressed in accordance with the present invention: ammonium thiocyanate, thiourea, dicyandiamide, hexamethylenetetramine, MASONEX (Trademark of Masonite Corporation), sucrose and mixtures thereof. Derivatives of urea such as guanamine, alkylureas (i.e. methylurea, ethylurea), lower alkylthioureas, methylolurea, biuret, inorganic acid salts of urea, and mixtures thereof are useful in forming skins. The remainder of the chemicals listed in Table II and mixtures thereof form weaker skins.

The chemicals listed in Table III were evaluated and found not to develop skins on the surface of low density consolidated mats when "post-consolidation" pressed at a temperature of at least 525° F.:

Table III LOW DENSITY, STRESSED-SKIN NON-SKIN PRODUCERS CHEMICAL

Ammonium Chloride

Ammonium Hydroxide

Dimethylol Urea

Aluminum Sulfate

Zinc Chloride (Hygroscopic)

Sodium Carbonate

Potassium Dichromate

Sodium Nitrate

Aniline

Melamine

Butylamine

Glycerin

Resorcinol

Formamide

Formic Acid

Ammonium Acetate

Raw Linseed Oil Without Dryer

Quaternary Ammonium Silicate

Starch

Hydrochloric Acid

Nitric Acid

Phosphoric Acid

In attempting to achieve thicker surface skins on the handleable mat during post-pressing it was found that the skin forming chemical should deeply penetrate the surface fibers of the handleable mat prior to post-pressing. Surface spraying therefore led to the impregnation method of incorporating the chemical into the handleable mat.

IMPREGNATION PROCESS

It has been found that when the skin forming chemicals are impregnated into the surface of a handleable mat after initial consolidation of that mat, a much thicker skin can be produced by vacuum impregnating the chemical to provide a skin thickness in the range of 0.060 inch.

In testing various degrees of chemical penetration by vacuum impregnating the handleable mat after initial consolidation, it was found that increasing the depth of chemical pentration beyond that required to produce 0.060 inch skins, for example, at a level of 100-150 pounds of chemical per thousand square feet, does not improve the physical properties of the panel, nor does it increase the skin thickness beyond 0.070 inches.

Typical post-consolidation press conditions an developing a skin in accordance with the present invention are as follows:

Platen Temperature--550° F. (Both Sides)

Closing Speed--Fast as Possible

Holding Time at Caliper--30 Seconds

Compress--0.100 inch

Pressure--200 p.s.i.

Opening Speed--Fast as Possible

The skin forming chemical can be included in overlay fibers in manufacturing the handleable mat. For example, a handleable mat can be produced by dry forming the mat in three layers from three separate head boxes with the skin forming chemical blended into the two surface layers, and consolidating to produce a handleable mat having the skin forming chemical in two distinct surface layers but not in the central core. The amount of chemical blended into the overlay or surface layers regardless of the method of manufacture should be at least 5% by dry weight of surface layer contacted with the chemical, and preferably in the range of 10-15% by dry weight of surface layer contacted. Resin binders in the surface layers can be reduced to about one half the normal amount (i.e. about 4%) since post-consolidation hot-pressing at a temperature of at least 525° F. substantially increase the strength of the surface layers.

To provide for better surface smoothness and paint holdout, a handleable mat produced as described by the overlay process can be surface treated with the skin forming chemical in an amount of about 5-15 pounds of skin forming chemical per thousand square feet by spraying an aqueous solution of the chemical onto the surface of the mat just prior to post-pressing.

In addition to being stiffer and stronger than hardboard having a much greater density, the boards having skins produced in accordance with the present invention are superior with respect to hardness, embossing fidelity, and paint holdout. "Design fidelity" or simply "fidelity" as used herein is a measure of the accuracy of reproduction of the design of an embossing plate onto the surface of a consolidated mat. The panel produced in accordance with the present invention permits deeper embossing patterns because of the lower density. "Paint holdout" is the ability of a panel to keep paint on its surface without a significant amount striking into the panel.

The amount of pressure used in post-pressing should not go beyond 300 p.s.i. so that a relatively low density core or central portion of the board can be maintained. Some caliper reduction must result during post-pressing to apply both heat and pressure necessary for thick skin development. Contact (unregistrable) pressure is sufficient for slight caliper reduction. It has been found that as the fibers are compressed during the post-pressing operation, the density increased in the overlay layers and, to some extent, in the core material. Thereafter, the core begins to resist compression. Accordingly, the face or overlay layers will compress more than the core.

POST-PRESS CONDITIONS

The handleable mat having a density of less than 35 pounds per cubic foot is "post-pressed" at a temperature of at least 525° F. To achieve the full advantage of the present invention, the temperature of "post-pressing" should not exceed 650° F. It is preferred to "post-press" at a temperature in the range of 525°-575° F. After chemical impregnation, the consolidated mat is generally dried to about 10% or less moisture prior to post-pressing.

To maintain a relatively low density core material in the fiberboard product, the pressure used in "post-pressing" should not exceed about 300 p.s.i. Pressures can vary depending upon the density of the handleable mat--with lower density mats (i.e. 10-20 pounds per cubic foot) it is preferred to use pressures on the order of 30-150 p.s.i. in post-pressing. It is always preferred to use a "post-press" pressure less than 200 p.s.i. The time of post-pressing is not critical and preferably is in the range of 5-60 seconds. With higher temperatures in the range of 575°-650° F., it is desirable to remove the product from the post-press operation within about 30 seconds to avoid charring or decomposition of the surface skins.

AMOUNT OF SKIN FORMING CHEMICAL

The amount of skin forming chemical necessary to form a sufficient skin thickness for the purpose of the present invention generally is at least about 5% by weight of the fibers contacted. Amounts vary depending upon the particular chemical, as set forth in Table II, to provide a relatively thick skin. The weight percent of skin forming chemical needed is at least 5% based upon the dry weight of the surface material in which the chemical has penetrated when combined by the impregnation process. When combined by the overlay process, the weight percent of skin forming chemical needed is at least 5% based upon the dry weight of overlay material.

To achieve the full advantage of the present invention, the amount of skin forming chemical incorporated into the handleable mat should be in the range of 6-20% based on the dry weight of the material contacted therewith, and preferably in the range of 9-17% by weight.

The product formed by the process of the present invention includes a hard, dense, smooth skin having a density greater than the density of the consolidated mat on which the skin is formed. The composition of the skins has not been determined by analysis. The base layer or core material underlying the skin has a density of 10-35 pounds per cubic foot, so that lightweight products of varying density can be manufactured having hard skins thereon.

Although the present invention is described in terms of particular constituents, and ranges thereof, and manner of making and using the same, it is recognized that departures may be made therefrom within the scope of the invention as defined in the appended claims. 

We claim:
 1. A fiberboard product comprising a base layer comprising cellulosic fibers, said base layer having a density in the range of 10-35 pounds/ft³ ; and an integral skin disposed on at least one face of said base layer, said skin having a higher density than that of the base layer and formed by contacting the surface fibers of said fiberboard with a skin forming chemical in an amount of 5-20% based on the dry weight of fibers contacted therewith, and heating said contacted surface fibers, under pressure, at a temperature of at least 525° F., said skin formed during said hot-pressing on the surface of said fiberboard contacted with said skin forming chemical.
 2. A method of manufacturing a cellulosic fiber containing board having a skin on at least one surface thereof comprising:compressing a fibrous composition, including cellulosic fibers, under heat and pressure to form a consolidated mat having a density of less than 35 pounds per cubic foot; including a skin forming chemical in at least a surface layer of fibers of said mat in an amount in the range of 5-20% based on the dry weight of said surface layers wherein said skin forming chemical is selected from the group consisting of ammonium thiocyanate, thiourea, acetamide, dicyandiamide, ethanolamine, hexamethylenetetramine, hydrazine+formaldehyde, ammonium xylene sulfonate, mono-ammonium phosphate, di-ammonium phosphate, ammonium sulfate, sucrose, boric acid+borax, linseed oil+iron tallate, a sugar containing hemicellulose extract derived from steam digestion of wood chips, and mixtures thereof; and hot-pressing said consolidated mat containing said skin forming chemical in at least its surface fibers, at a temperature of at least 525° F. to form a board having a skin on at least one surface thereof, said skin defined by a layer of material on said surface of said board having a density greater than the material on which the skin is formed, said skin formed during said hot-pressing on the surface of said mat containing said skin forming chemical.
 3. A method as defined in claim 2 wherein said skin forming chemical-containing mat is hot-pressed at a temperature in the range of 525-650° F.
 4. A method of forming a cellulosic fiber containing board having a relatively dense skin on at least one of its surfaces comprising:impregnating a consolidated cellulosic fiber containing mat, having a density of less than 35 pounds/ft³, with a skin forming chemical in an amount in the range of 5-20% based on the dry weight of the mat material contacted therewith, wherein said skin forming chemical is an amine or amide selected from the group consisting of thiourea, dicyandiamide, ethanolamine, hexamethylenetetramine, and hydrazine plus formaldehyde; and pressing said impregnated mat at a temperature of at least 525° F., said skin formed during said hot-pressing on the surface of said mat impregnated with said skin forming chemical.
 5. A method as defined in claim 2 wherein said skin forming chemical is an ammonia liberating chemical selected from the group consisting of ammonium thiocyanate, ammonium xylene sulfonate, mono-ammonium phosphate, di-ammonium phosphate and ammonium sulfate.
 6. A method as defined in claim 2 wherein said skin forming chemical comprises a sugar selected from the group consisting of sucrose, xylose, glucose, mannose, galactose, arabinose, and mixtures thereof.
 7. A method as defined in claim 2 wherein said skin forming chemical comprises linseed oil catalyzed with iron tallate.
 8. A method as defined in claim 2 wherein said skin forming chemical is a combination of boric acid and borax.
 9. In a method of manufacturing a fiberboard, including the steps of hot-pressing a cellulosic fiber-containing mat to consolidate said mat into a unitary structure having a density less than 35 pounds per cubic foot, the improvement comprising impregnating at least one surface of said consolidated mat with a skin forming chemical in an amount of at least 5% based on the dry weight of the portion of the mat contacted with said chemical, and thereafter hot-pressing said impregnated, consolidated mat at a temperature of at least 525° F. for a period of time sufficient to form a hard, dense skin on the surface of said mat, said skin formed during said hot-pressing on the surface of said mat impregnated with said skin forming chemical.
 10. A method of forming a hard, dense skin on the surface of a fiberboard comprising:contacting the surface fibers of a cellulosic fiber-containing mat, having a density less than 35 pounds per cubic foot, with a skin forming chemical selected from the group consisting of ammonium thiocyanate, thiourea, di-cyandiamide, hexamethylenetetramine, a sugar containing hemicellulose extract derived from steam digestion of wood chips, sucrose and mixtures thereof; and thereafter hot-pressing said mat in a heated press at a temperature of at least 525° F. to form a board having a skin on said surface of said mat, having a density greater than the density of material on which the skin is formed, said skin formed during said hot-pressing on the surface of said mat contacted with said skin forming chemical.
 11. A method as defined in claim 10 wherein said mat is hot-pressed at a temperature in the range of 525°-650° F. and at a pressure not exceeding 300 p.s.i.
 12. A method of manufacturing a cellulosic fiber-containing board having a skin on at least one surface thereof comprising:depositing a first layer of fibers to form a base layer for supporting at least one surface skin thereon; depositing a second layer of fibers over said first layer of fibers, said second layer of fibers including a skin forming chemical in an amount of at least 5% based on the total weight of said second layer of fibers, wherein said skin forming chemical is a urea derivative selected from the group consisting of guanamine, alkylureas, lower alkylthioureas, biuret, inorganic acid salts of urea, and mixtures thereof; consolidating said first and second layers in a heated press at a temperature less than 500° F. and at a pressure less than 700 p.s.i. to form a consolidated, handleable mat having a density in the range of 10-35 pounds/ft³ ; and thereafter hot-pressing said consolidated, handleable mat at a temperature in the range of 525°-650° F. and at a pressure not exceeding 300 p.s.i. to form at least one skin on said base layer, said skin formed during said hot-pressing on the second layer of fibers including said skin forming chemical.
 13. A method as defined in claim 12 further including depositing a third layer of fibers in contact with said base layer, prior to consolidation of said first and second layer to thereby sandwich said base layer between said second and third layers, said third layer of fibers including a skin forming chemical in an amount of at least five percent based on the total weight of said third layer of fibers wherein said skin forming chemical included in said third layer is also selected from said group of urea-derivatives; and consolidating said three layers to a density in the range of 10-35 pounds/ft³.
 14. A method of manufacturing a cellulosic fiber-containing board having a skin on at least one surface thereof comprising:depositing a water slurry of cellulosic fibers onto a water pervious support member to form a wet mat; drying said wet mat to less than 8% water, based on the total weight of said mat, to consolidate said mat, wherein said consolidated mat has a density of less than 35 pounds per cubic foot; including a skin forming chemical in at least the surface layer of fibers of said mat in an amount in the range of 5-20% based on the dry weight of said surface layer wherein said skin forming chemical is an amine selected from the group consisting of thiourea, ethanolamine, hexamethylenetetramine, hydrazine plus formaldehyde and mixtures thereof; and hot-pressing said consolidated mat containing said skin forming chemical in at least its surface fibers, at a temperature of at least 525° F. to form a board having a skin on at least one surface thereof, said skin defined by a thickness of material on said surface of said board having a density greater than the material on which the skin is formed, said skin formed during said hot-pressing on the surface layer of said mat containing said skin forming chemical.
 15. A method of manufacturing a mineral wool board having a skin on at least one surface thereof comprising:forming a consolidated mineral wool fiber-containing handleable mat, having a density in the range of 10-35 pounds per cubic foot and having a cured thermosetting resin binder; including a skin forming chemical in at least a surface layer of fibers of said mat in an amount in the range of 5-20% based on the dry weight of said surface layer wherein said skin forming chemical is selected from the group consisting of ammonium thiocyanate, thiourea, acetamide, dicyandiamide, ethanolamine, hexamethylenetetramine, hydrazine+formaldehyde, ammonium xylene sulfonate, mono-ammonium phosphate, di-ammonium phosphate, ammonium sulfate, a hydrolyzed hemicellulose extract containing 35% simple sugars having the following distribution:14.0% Glucose, 27.0% Mannose, 8.0% Galactose, 5.0% Arabinose, 46.0% Xylose, sucrose, boric acid+borax, linseed oil+iron tallate, and mixtures thereof; and hot-pressing said consolidated mat containing said skin forming chemical and said cured thermosetting resin binder, at a temperature of at least 525° F. to form a board having a skin on at least one surface thereof said skin formed during said hot pressing on the surface of said board contacted with said skin forming chemical.
 16. A method as defined in claim 15 wherein said skin forming chemical is included in said mat prior to consolidation thereof.
 17. A method as defined in claim 15 wherein said skin forming chemical is included in said mat after consolidation thereof.
 18. A method of manufacturing a cellulosic fiber-containing board having a skin on at least one surface thereof comprising:depositing a layer of fibers including cellulosic fibers onto a support member to form a loose mat containing cellulosic fibers; consolidating said mat to bond said fibers together to form a completed board; contacting at least a surface of said completed board with a skin forming chemical selected from the group consisting of ammonium thiocyanate, thiourea, acetamide, dicyandiamide, ethanolamine, hexamethylenetetramine, hydrazine+formaldehyde, ammonium xylene sulfonate, mono-ammonium phosphate, di-ammonium phosphate, ammonium sulfate, sucrose, boric acid, linseed oil+iron tallate, and mixtures thereof in an amount sufficient to form a higher density surface skin when said contacted surface is hot-pressed at a temperature of at least 525° F.; and hot-pressing said contacted surface at a temperature of at least 525° F. to form a skin on at least one surface thereof, said skin formed during said hot-pressing on the surface of said board contacted with said skin forming chemical.
 19. A method as defined in claim 18 further including drying said skin forming chemical contacted mat prior to hot-pressing said contacted surface.
 20. A method of manufacturing a cellulosic fiber-containing board having a skin on at least one surface thereof comprising:depositing a layer of fibers including cellulosic fibers onto a support member to form a loose mat containing cellulosic fibers; contacting at least a surface of said mat with a skin forming chemical selected from the group consisting of ammonium thiocyanate, thiourea, acetamide, dicyandiamide, ethanolamine, hexamethylenetetramine, hydrazine+formaldehyde, ammonium xylene sulfonate, mono-ammonium phosphate, di-ammonium phosphate, ammonium sulfate, sucrose, boric acid, linseed oil+iron tallate, and mixtures thereof in an amount sufficient to form a higher density surface portion when said contacted surface is hot-pressed at a temperature of at least 525° F.; consolidating said skin forming chemical contacted mat to bond said cellulosic fibers together to form a completed board; and hot-pressing said completed board at a temperature of at least 525° F. to form a skin on at least one surface thereof, said skin formed during said hot-pressing on the surface of said board contacted with said skin forming chemical.
 21. A method as defined in claim 20 further including contacting a surface of the consolidated board with additional skin forming chemicals prior to hot-pressing at a temperature of at least 525° F. 